Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to apparatus for sensing torque applied to a torsion member, such as a shaft of a motor or a power transmission system.
2. Description of the Related Art
It often is desirable to measure the torque applied by a shaft or other type of torsion member. For example, in motor vehicles, a torque sensor is mounted on the shaft to which the steering wheel is attached in order to measure the amount of force that the driver is using to turn the wheels of the vehicle.
Heretofore, torque transducers often employed strain gauges mounted on the shaft to detect deflection of the shaft that resulted from application of torque. Strain gauges commonly are used to measure deflection of a structural member caused by forces acting on that member. The resistance of the Strain gauge changes upon being physically deformed. The electrical leads of the strain gauge are connected to a resistor bridge, such as a Wheatstone bridge, which produces an output signal that varies with variation of the torque applied to the shaft.
Another type of a torque sensor uses magneto-elastic elements fixedly positioned on the shaft and electromagnetic sensing coils that detects the change in the magnetic field as the magneto-elastic element is subjected to torsional strain. It was common to encounter difficulty in attaching the magneto-elastic elements to the shaft in a manner which would reliably convey the strain to the magneto elastic element. This was a drawback to mass production of such torque sensors.
Other force transducers employ a piezoelectric element mechanically fastened to the member in which the strain is to be detected. The piezoelectric element produces an electric charge output when the transducer is subjected to a high impulse load. Such a device, as described in U.S. Pat. No. 4,835,436 issued to H. S. Lew, is not well suited in applications where a low torsional strain rate, or a low magnitude torque is applied to the member. Thus this device is unsuitable for automotive steering shaft applications.
Therefore, it has long been desired to provide a way or means for sensing torsional strain in a member subjected to an applied torque in a manner that provides a high degree of sensitivity. Such a desire also involves the need to produce an electrical signal which indicates the torsional strain and which has a high resolution in response to changes in the torsional strain of the member. It has been sought to provide such a torque sensing mechanism which is immune from effects of environmental conditions, such as temperature and pressure variations.
These and other desires are satisfied by an apparatus for measuring torque in a torsion member, that apparatus includes a transducer adapted to be attached to the torsion member. The transducer has a piezoelectric element, such as a quartz crystal, that has a first resonant frequency (fTRANSDUCER) which varies with variation of strain in the torsion member. A transducer oscillator is connected to the transducer and produces a first signal with frequency that is a function of the first resonant frequency. In the preferred embodiment of the present apparatus, the frequency of the first signal is a harmonic of the first resonant frequency.
A reference oscillator includes a reference piezoelectric element having a second resonant frequency (fREFERENCE) The reference oscillator produces a second signal having a frequency that is a function of the second resonant frequency. A mixer is connected to the transducer oscillator and to the reference oscillator to heterodyne the first signal and the second signal thereby producing a resultant signal. In the preferred embodiment, the resultant signal is low pass filtered to remove frequency components other than the frequency difference between the first and second signals.
Because the resonant frequency of the transducer changes with the variation of the strain in the torsion member, the frequency of the first signal indicates the torque applied to the torsion member. However, the frequency of the first signal also may vary due to changes in temperature, pressure, age, and other environmental effects. To counter those effects, the reference piezoelectric element is provided that has a resonant frequency that varies with environmental effects in the same manner as the transducer. Therefore, mixing the first and second signals cancels the environmental effects and produces the resultant signal having a frequency that varies only as a function of the torque on the torsion member.
A novel mechanism for capacitively coupling the transducer on a rotating torsion member to the transducer oscillator also is described.